Using DNA analysis, scientists have found that most red snapper sold at grocery stores in the United States may not be red snapper after all.

They estimate that three-quarters of fish labeled as the popular and tasty red snapper are lesser-known species of snapper or entirely different species of fish. Only the species Lutjanus campechanus can be legally designated the common name “red snapper,” according to the U.S. Food and Drug Administration.

The mislabeling of fish is a violation of federal law, but it is unclear whether the mislabeling is deliberate or whether it occurs in retail stores, on fishing boats and docks, or somewhere in between.

The study began as a class project for students studying population genetics at University of North Carolina in Chapel Hill, and both the teacher and students were entirely surprised by the extent of the abuse.

“We had no idea of the scale of mislabeling of red snapper,” says Peter Marko, who led the study at UNC. “We looked at red snapper because we heard anecdotal stories about substitution of other fish for red snapper at restaurants,” he adds.

Marko and his students analyzed stretches of DNA from fish fillets sold at nine grocery stores in eight states including Delaware, New York, South Carolina, North Carolina, and Florida. The students collected fish fillets on trips home from school, and in one case a student had her parents ship a fish from Massachusetts.

They compared DNA sequences of the grocery store fish to DNA sequences of red snapper available online from previous studies and found that 17 of 22 fish sold as red snapper were other species of fish. Some of these fish were vermillion or lane snapper from the Atlantic Ocean, and more than half were from other parts of the world or were rare species.

The findings could signal dwindling stocks of red snapper, the researchers warn. In addition to misleading consumers about what they’re eating, the abundance of fish posing as red snapper at marketplaces might give the public the impression that red snapper is plentiful. Estimates of the actual size of red snapper fisheries could also be way off.

Red snapper, which is found in the Atlantic Ocean, is one of the most important fisheries in the Gulf of Mexico, but over fishing led to stricter U.S. regulations for catches in the late 1990s. Meanwhile consumer demand for the fish had continued to rise.